There are several factors that affect the performance of your backup tasks. Here we describe the most common conditions that affect backup performance, and offer some suggestions for mitigating the effects of those conditions.
Reduce the number of files considered for backup
CCC analyzes all of the files that are included in your backup set for consideration to be copied. If you have a particularly high number of files on your source volume, you may want to put some thought into how your files are organized. For example, if you have a large number of files that never change (perhaps some old, completed projects), you can collect these into a folder named "Archives", back it up once, then exclude it from future backups. CCC will not delete excluded items from your destination (unless you ask it to using Advanced Settings), so as long as you keep the original on your source volume, you will always have two copies of your archived content. Because these items are excluded from your daily backups, CCC will not spend time or RAM enumerating through those files for changes.
Performance will be worse for smaller rotational hard drives (e.g. physically smaller, like those in 2.5" hard drive enclosures), for older hard drives, and for hard drives that are nearly full and thus more likely to be fragmented. You will also get longer copy times when you have lots of small files vs. a volume filled with just a few very large files. Finally, you will see better performance with faster/more efficient interfaces — Thunderbolt is faster than Firewire, Firewire 800 is faster than USB 2.0, etc.
When you consider purchasing an external hard drive for backup, we recommend enclosures that have multiple interfaces (e.g. Firewire and USB, or Thunderbolt and USB). Depending on how you use the Firewire or USB interfaces on your Mac, you may find that you get better performance or reliability when trying a different interface on your external backup disk. Additionally, if your source volume is nearly full, we recommend that you replace it with a larger hard drive to avoid the performance implications of filesystem fragmentation.
It's important to choose the right filesystem for the hardware that you have. If you have an older, rotational HDD, you should format that device using the "Mac OS Extended, Journaled" (HFS+) format. APFS is the new, modern standard, but its performance on rotational devices is inferior to HFS+. Apple still recommends HFS+ for rotational HDD devices, and that's our recommendation as well.
Anything that causes CCC to compete for bandwidth to your source or destination volume will increase the amount of time that it takes to back up your data. Spotlight indexing is one such process that CCC typically must compete with for disk bandwidth. As you copy new data to your destination volume, for example, Spotlight wants to read those "new" files so it can index their contents. Having a Spotlight index of your backup volume may be unnecessary as you probably want to search for files only on your source volume. To disable Spotlight indexing on a volume that is dedicated to backup, drag the icon of the destination volume into the "Privacy" tab of Spotlight Preference Pane in the System Preferences application. If you do want the backup volume indexed, drag its icon out of the "Privacy" tab after the cloning and indexing will start immediately.
CCC offers an advanced option to "Find and replace corrupted files". When using this option, CCC will re-read every file on the source and every file on the destination, calculating a checksum of each file. CCC then compares these checksums to see if a file should be recopied. While this is an excellent method for finding unreadable files on the source or destination, it will dramatically increase the amount of time that your backup task takes, and it will also increase CPU and hard drive bandwidth consumption on your Mac. We recommend limiting the use of this option to weekly or monthly, and scheduling such tasks to run when you are not typically using your Mac.
In fact it's unbelievably slow. If you attach an SSD-bearing Mac in Target Disk Mode to another Mac via a USB-C cable (so both at 10Gb/s connections), you might expect to get incredible speed (e.g. >500MB/s). You will be sorely disappointed by speeds of less than 20MB/s; slower than USB 2.0. For better performance, we recommend that you avoid Target Disk Mode. Boot the target Mac from the volume you're trying to restore instead. Not only will you get better performance, but you also have the assurance that the Mac can boot from the OS that you're restoring to it.
Other applications and conditions that can lead to performance problems
Over the years we have received numerous queries about poorer performance than what is expected. Careful analysis of the system log and Activity Monitor will usually reveal the culprit. Here are some things that we usually look for:
- Other backup software copying simultaneously to the same volume, a different volume on the same disk, or across the same interface as CCC's destination.
- Utilities that watch filesystem activity and do things when file changes are detected. Antivirus software is a common culprit, but we have also seen problems caused by other watcher applications, such as memeod and Western Digital's SmartWare.
- Slow interfaces — USB hubs (including the ports on a USB keyboard or display) and even some USB cables can reduce the bandwidth to your disk dramatically. If you're using USB, be sure that your device is plugged directly into one of the USB ports on your Mac.
- Daisy chaining Firewire devices is usually OK, though some enclosures can stall the entire Firewire bus when given too much bandwidth. If you see this behavior, try switching the order of devices in the chain, or attach your backup disk directly to a Firewire port on your Mac.
- Using a wireless network connection to connect to a network volume. If you're seeing poor performance with a wireless connection, compare the performance when using a wired (ethernet) connection.
Use the Console application to view the contents of the system log. If you're still having trouble identifying a performance problem, we're here to help.